AB1455 FLUORIDE UPTAKE ON QUANTITATIVE POSITRON EMISSION TOMOGRAPHY/COMPUTED TOMOGRAPHY (NA18F-PET/CT) AS NOVEL BIOMARKER FOR DISEASE ACTIVITY IN ADULT CHRONIC NONBACTERIAL OSTEOMYELITIS/SYNOVITIS ACNE PUSTULOSIS HYPEROSTOSIS OSTEITIS SYNDROME (CNO/SAPHO)
Authors: A. Leerling, F. Smit, Z. Späth, A. Navas Cañete, L. F. De Geus-Oei, A. Van de Burgt, W. Van der Bruggen, O. Dekkers, N. Appelman-Dijkstra, D. Vriens, E. Winter
Journal: Annals of the Rheumatic Diseases (2023)

<h3>Background</h3> Chronic nonbacterial osteomyelitis (CNO) is a rare bone disease characterized by sterile bone inflammation and locally increased bone turnover. CNO that occurs in the rheumatic spectrum of synovitis, acne, pustulosis, hyperostosis and osteitis is referred to as SAPHO syndrome [1, 2]. To date, there are no objective biomarkers for monitoring disease activity. Recommended imaging tools like technetium radiolabeled hydroxymethylene diphosphonate single positron emission computed tomography visualize the increased bone turnover, but fail to reflect the relapsing-remitting disease activity course due to an imprinting pattern [3]. Likewise, magnetic resonance imaging can track the fluctuation of bone marrow edema, but is inferior compared to computed tomography (CT) in capturing subtle inflammatory and accumulated structural changes that reflect disease duration. Sodium fluoride-18 positron emission tomography/CT(Na<sup>18</sup>F-PET/CT) is another imaging modality which produces quantitative data on the spatial distribution of bone turnover that also correlate with clinical disease activity in other metabolic bone diseases [4]. <h3>Objectives</h3> We quantified <sup>18</sup>F-uptake in CNO/SAPHO patients to evaluate its capacities as a disease activity biomarker. <h3>Methods</h3> Cohort study including 43 CNO/SAPHO patients not using immunomodulatory or antiresorptive medications with Na<sup>18</sup>F-PET/CT performed at our expert clinic between 2019-2022. Images were qualitatively assessed following a systematic reporting format. Maximal standardized uptake values (SUV<sub>max</sub>) were determined in the following areas as defined by radiologic appearance on CT: osteitic areas (osseous lesions), inflamed ligaments or joint spaces (soft tissue lesions) and thoracic vertebrae 5 as reference bone (ref-bone). <h3>Results</h3> Qualitative assessment revealed sclerosis and hyperostosis of the manubrium as most prevalent radiologic features (present in 77% and 70%) and costae 1-2 (79% each). Calcification/ankylosis of the costoclavicular ligament was seen in 58%. Quantitative analyses showed higher SUV<sub>max</sub> of osseous and soft tissue lesions compared to ref-bone (mean 22±6 and 26±9 vs. 11±3 g/mL, <i>p&lt;0.001</i> for paired difference). Stratified per location, highest SUV<sub>max</sub> was found in soft tissue lesions of the costoclavicular ligament followed by the manubriosternal joint (mean SUV<sub>max</sub> of 29±13 and 25±8 respectively). SUV<sub>max</sub> in soft tissue lesions positively correlated with erythrocyte sedimentation rate (Spearman correlation coefficient 0.546, <i>p&lt;0.01</i>). <h3>Conclusion</h3> CNO/SAPHO patients display distinctive qualitative imaging features that are well-captured on Na<sup>18</sup>F-PET/CT, and also show higher SUV<sub>max</sub> in osseous and soft tissue lesions compared to ref-bone. Diseased soft tissue lesions showed highest SUV<sub>max</sub>. Considering Na<sup>18</sup>F precipitates in young osteoid, these high values may reflect the process of soft tissue ossification that is common in long-existing CNO/SAPHO, as also demonstrated in fibrodysplasia ossificans progressiva [5]. Further studies should evaluate the promising application of quantified uptake on Na<sup>18</sup>F-PET/CT as a novel biomarker for disease activity and therapy response in CNO/SAPHO. <h3>References</h3> [1]Leerling et al. Clinical and therapeutic diversity in adult chronic nonbacterial osteomyelitis (CNO) of the sternocostoclavicular region: a meta-analysis. Rheumatology 2022. [2]Rukavina I. SAPHO syndrome: a review. J Child Orthop 2015. [3]Li et al. A retrospective study of bone scintigraphy in the follow-up of patients with SAPHO syndrome: is it useful to repeat bone scintigraphy for disease assessment? Clinical Rheumatology 2019. [4]van der Bruggen et al. Quantifying skeletal burden in fibrous dysplasia using sodium fluoride PET/CT. Eur J Nucl Med Mol Imaging 2020. [5]Botman et al. Evolution of heterotopic bone in fibrodysplasia ossificans progressiva: An [(18)F]NaF PET/CT study. Bone 2019. <h3>Acknowledgements:</h3> NIL. <h3>Disclosure of Interests</h3> None Declared.